1. Field of the Invention
The invention relates to a surface activating process which is used to improve the surface characteristic.
2. Description of the Related Art
Among liquid crystal display devices there is, on the hand, a liquid crystal display devices of the transmission type, which is formed of a liquid crystal panel, a driver to control the latter, and a back light. On the other hand, there is a liquid crystal display of the reflection type which does not use back light, but internal lighting.
A liquid crystal cell conventionally consists of two substrates. On one of these substrates, a driver system for triggering the liquid crystals, such as a thin-film transistor, and electrodes for triggering the liquid crystals, as well as an alignment layer for alignment of liquid crystals to a certain direction, are formed. A light screening film which is called a black matrix is formed on the other of these substrates. In a color liquid crystal cell, a color filter and the above described alignment layer are formed.
The alignment layer is produced by subjecting one surface of a thin film, such as polyimide resin or the like, to a treatment which is called rubbing, and by providing it with fine grooves (scratches) in a certain direction; this is intended to align the molecules of the liquid crystal along these fine grooves in a certain direction.
In a liquid crystal cell, the picture contrast changes according to the angle of view field The angular range in which advantageous contrast is obtained is called the angle of view field of the liquid crystal cell. The greater this angle of view field, the more visible and better the liquid crystal cell becomes. Therefore, there is the important technical object of how a large visual angle is obtained.
In a known process, a pre-tilt angle of the molecules of the liquid crystals is controlled. Here, the term "pre-tilt angle" is defined as an angle for which the molecules of the liquid crystals with reference to the surface of the alignment layer rise with a certain angle when they come into contact with the alignment layer. In the process, the alignment layer is irradiated through a mask with ultraviolet light, and thus, the characteristic of the alignment layer is activated. This technique of controlling the pre-tilt angle is known, for example, from Japanese patent disclosure document HEI 6-222366 and Japanese patent disclosure document HEI 6-281937.
Here, a liquid crystal cell with a wide the angle of view field is produced using the phenomenon that the pre-tilt angle of the areas irradiated with the ultraviolet light decreases and using the measure by which, within this same substrate, areas with different pre-tilt angles are arranged in mixed fashion.
The phenomenon that the pre-tilt angle is changed by irradiation with ultraviolet light is presumably caused by the fact that the surface of the alignment layer is oxidized by the ultraviolet light and that the polarity of the alignment layer changes. To activate the above described alignment layer it is necessary to place the alignment layer within a gas atmosphere which contains oxygen when it is irradiated with ultraviolet light. This means that, in irradiation of the alignment layer with ultraviolet light through the mask, the mask and the surface of the alignment layer (substrate surface) are held at a distance from one another and a gas layer (air) which contains oxygen is formed between the mask and alignment layer.
FIG. 8 is a schematic of the known process for activating the characteristic of the alignment layer by irradiation with ultraviolet light in the above described manner.
On mask MK, a mask pattern MP is formed which partially screens the ultraviolet light. On the substrate 101, the above described thin-film transistor, the above described electrodes for triggering of the liquid crystals and the like, which are not shown in the drawing, are formed and on which alignment layer 102 is formed.
To improve the characteristic of alignment layer 102, as shown in FIG. 8(a), the mask MK, on which the mask pattern MP is formed, is positioned at a distance from the substrate 101 on which alignment the layer 102 is formed, and the mask MK is irradiated with ultraviolet light from above. In this way, as shown in FIG. 8(b), the characteristic of alignment layer 102 is partially activated and the pre-tilt angle of the areas irradiated with the ultraviolet light changes.
To activate the characteristic of the above described alignment layer, ultraviolet light with wavelengths of less than or equal to 300 nm (especially from 200 nm to 300 nm) can be used. A mercury lamp of the long arc type is known as a light source which emits light in this wavelength range. The reason for this is that the distance between the discharge electrodes is great, i.e., it is a few dozen cm to a few hundred cm, that the surface of the emission part is large, and that therefore strong ultraviolet light is easily obtained.
As was described above, a mercury lamp of the long arc type is known as a lamp which is used to activate the characteristic of the alignment layer or the like which is formed on the substrate. In the following, the substrate is called a workpiece. The ultraviolet light from a lamp of this type is, however, scattered light.
If irradiation is performed with ultraviolet light in the state in which the mask and the workpiece surface are separated from one another, as was described above, the component of the light which is obliquely incident on the mask enters the interior of the light screening area of the mask. As a result thereof, areas which are not actually to be activated are irradiated with ultraviolet light and activation of undesired areas is effected. If, for example, in the above described FIG. 8, scattered light is used, ultraviolet light which is obliquely incident enters the areas which are not to be irradiated with ultraviolet light; this causes a change of the pre-tilt angle of the areas in which a change of the pre-tilt angle is actually not wanted.
If, on the other hand, the distance between the mask and the workpiece surface is reduced, or if the two are brought into contact with one another in order to prevent the above described entry of the oblique light, the disadvantage arises that, as a result of the lack of a sufficient amount of oxygen for the activating reaction, sufficient activation cannot be achieved.
To prevent the aforementioned activation of the undesired areas, therefore, it is necessary to irradiate the mask as well as the workpiece with parallel light using an optical system.
If a lamp is used with an arc length such as in a mercury lamp of the long arc type or the like, using a lens with a high degree of parallelism of the central light beam, the degree of parallelism of the light emitted from the middle area of the lamp can be increased. The light which is emitted from the end of the lamp is, however, obliquely incident on the workpiece, this angle being designated the visual angle. The visual angle is defined as a half angle with consideration of a light source. As a result thereof, advantageous parallel light is not obtained and the oblique light enters.
Furthermore, a lamp with a large arc length has a large discharge surface. When an integrator lens or the like is used, it is necessary to enlarge this integrator lens. This leads to the disadvantage of resultant higher costs. In this case, the "are length" is equivalent to the distance between the electrodes of the lamp.
If, on the other hand, a lamp is used with a short arc length, such as a super high pressure mercury lamp or the like, the degree of parallelism of the light can be increased and the visual angle can be decreased. If the visual angle is extremely decreased, light diffraction phaenomenon occurs more. As a result, the exposure of the screened parts increases due to the diffracted light.
Furthermore, a lamp with a short arc length is constructed such that the light with a so-called i-line (365 nm), an h-line (405 nm) and a g-line (436 nm) is emitted with high efficiency. Radiation of the above described ultraviolet light with wavelengths of less than or equal to 300 nm is low in this case. In a lamp with a low arc length (discharge lamp of the short arc type), therefore, in the wavelength range which is necessary for activating the surface of the alignment layer, a irradiance in a sufficient amount cannot be obtained. In addition, when used as a surface activation lamp, it is considered disadvantageous that the duration of irradiation becomes long.